Please refer to FIG. 1 which is a circuit diagram of a frequency converter of the prior art. The frequency converter includes a power source 15, an alternating-current/direct-current (AC/DC) converter 11, a direct-current/alternating-current (DC/AC) converter 12, and a current sensor 14. The output signal of the frequency converter is an alternating-current signal and is outputted to a load 13. The DC/AC converter 12 includes a plurality of switching circuits which will generate many harmonic waves and in turn adversely affect the characteristics of the power signal, i.e. the power factor. The power factor is a ratio of the actual power to apparent power (simple product of voltage and current). The power factor p can be calculated from an equation of p=cos .theta., where .theta. is the phase angle of the signal. In order to maintain the current's stability of the load 13, the harmonic waves should be eliminated and a method for compensating the signal according to the power factor is needed. Therefore, in order to estimate a compensating factor of the signal, the signal outputted from the DC/AC converter 12 is measured by the current sensor 14 and then transmitted to a circuit, e.g. a microprocessor, for processing and compensating the signal.
The current sensor 14 positioned at the alternating-current end of the DC/AC converter 12 must have an ability for loading large electric powers. However, the current sensor 14, e.g. current transformer (CT), is relatively expensive. In order to reduce the cost of the converter, the current sensor 14 is removed and the method for estimating the compensating factor should be changed.
Because the switching circuits inside the DC/AC converter 12 will affect the power signal, the output current transmitted to the load can be estimated by the input current of the DC/AC converter 12. Please refer to FIG. 2 which shows the circuit of another frequency converter of the prior art. A resistor 21 is placed at the direct-current end of the DC/AC converter 12 as shown in FIG. 2. The resistor 21 is used as a feedback device for compensating the output signal of the DC/AC converter. The signal is compensated according to the phase angle of the output signal, a compensating factor is then calculated. The compensating factor for compensating the output signal of the DC/AC converter is estimated according to the signal passing through the resistor 21. This compensating method is a dead-time compensation.
Please refer to FIG. 3 which shows the circuit used in the dead-time compensation of the prior art. The voltage difference of the resistor 21 is measured and then transformed to a current signal. The current signal is amplified by a preamplifier 31 and then divided into a positive peak signal and a negative peak signal. The positive peak current I.sub.peak (+) and negative peak current I.sub.peak (-) are respectively measured by the positive peak current detector 32 and the negative peak current detector 33. Both I.sub.peak (+) and I.sub.peak (-) are compared by a comparator 34. The relatively larger one of the I.sub.peak (+) and I.sub.peak (-) is defined as an estimated output current and then outputted to a microprocessor 35 for calculating the compensating factor of the dead-time compensation.
Please refer to FIG. 4 which is a flow chart showing the method for compensating the signal. First, the output voltage V.sub.o of the load is defined and the input voltage V.sub.i and input current I.sub.i of the DC/AC converter are also measured. Thereafter, the positive peak current I.sub.peak (+) and the negative peak current I.sub.peak (-) of the feedback device, i.e. the resistor, are measured. The relatively larger one of I.sub.peak (+) and I.sub.peak (-) is defined as an estimated output current I.sub.o. The compensating factor d for compensating the output signal is calculated by an equation of ##EQU1## The output current of the DC/AC converter 12 is adjusted according to the compensating factor d.
However, the waveform of the signal outputted from the DC/AC converter 12 is varied along with different phase angles. Please refer to FIGS. 5(a)-5 (c) which show different waveforms with different phase angles. FIGS. 5(a), 5(b), 5(c) are diagrams respectively show the waveform of the output signal having a phase angle of 30, 60, 90 degree. The waveform of the output signal is changed when the phase angle is greater than 30 degree because the estimated output current I.sub.o, i.e. the relatively larger one of I.sub.peak (+) and I.sub.peak (-), is not the same as the actual output current. In fact, when the phase angle is greater than 60 degree, the estimated output current I.sub.o is smaller than the actual output current. Therefore, the compensating factor d calculated by the estimated output current I.sub.o is not correct, and this method for compensating the output signal is poor. Besides, the estimated output current I.sub.o is also smaller than the actual one when the phase angle is ranged from 30 to 60 degree. It is not easy to calculate an estimated output current which approaches to the actual current unless the current is measured by a detector with a high sensitivity. However, the detector with a high sensitivity is very expensive and the cost of the converter will be increased.
Referring to FIG. 6, the form of the output current with poor compensation is not smooth and will cause a damage of the load. The operation of the load is very unstable. Therefore, it is tried by the applicant to solve the above-described problem.